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PCB pattern for René Schmitz "late" MS20 VCF?
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fonik



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PostPosted: Mon Dec 10, 2007 5:19 am    Post subject: Reply with quote  Mark this post and the followings unread

slightly OT, but i finally decided to post four samples here re the resonance/self oscillation of my wasp clone (edit), interested in your comments.


so1.mp3
 Description:
sample #1: selfoscillating, no input signal, 1n4148 diodes in the feedbackpath (i did not normalize the sample to show the difference to sample #2

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so2.mp3
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sample #2: same as #1 but with green LEDs.

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saw_rm1_2.mp3
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sample #3: saw wave input, resonnance fully turned on, 1n4148 first, then green LEDs.

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sq_rm1_2.mp3
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sample #4: same as #3 but with square wave input...

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 Filename:  sq_rm1_2.mp3
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frijitz



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PostPosted: Mon Dec 10, 2007 8:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

So -- I have this HP extended to third order. I will have outputs for all three stages, since they all sound different, especially with the anharmonic oscillations. The 3d order has nice sharp resonances when working as a filter.

When using with the distortion/oscillation/drive the 2nd order seems to have more of the anharmonic/chaotic sounds. These seem harder to get in 3d order for some reason, but there are still lots of nice ones.

For clipping I have settled on using back-to-back 7.5V Zeners, which give a nice limiting for the oscillator mode. In addition I will have a switch to add a LED across them for asymmetric distortion.

Tomorrow I start final tweaking, including addition of VC resonance, compensation for the high-frequency Q enhancement and a careful look at DC vs AC coupled operation.

Very Happy

Ian
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frijitz



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PostPosted: Tue Dec 11, 2007 4:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
... At the point where resonance becomes fairly apparent with a signal present, removing the signal will leave the filter self-oscillating. In fact, I use that for effect now and then by just cranking the input level all the way down.

Scott -- Another question. Can you get the same effects in LP mode?

Very Happy

Ian
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v-un-v
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PostPosted: Tue Dec 11, 2007 4:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
slightly OT, but i finally decided to post four samples here re the resonance/self oscillation of my wasp clone (edit), interested in your comments.


3 & 4 sound like an MS20. There's no doubt about that. BUT it also reminded me of what a pig the MS20 is during live performances- simply because the filter is so hard to tame at times.

I always thought 10 turn MS20 resonance pots would be a good idea.

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PostPosted: Tue Dec 11, 2007 4:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

ahh- Wasp clone?? Surprised


Cool

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Scott Stites
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PostPosted: Tue Dec 11, 2007 10:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Dang, how did I lose track of this thread? Yep, Ian - LP mode will do the same thing. You can have a fair amount of resonance going, remove the signal, and the filter will be self oscillating.

I gotta download Fonik's examples - I've never heard a Wasp filter in the flesh.

Cheerio,
Scott

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frijitz



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PostPosted: Wed Dec 12, 2007 8:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
Dang, how did I lose track of this thread? Yep, Ian - LP mode will do the same thing. You can have a fair amount of resonance going, remove the signal, and the filter will be self oscillating.

I gotta download Fonik's examples - I've never heard a Wasp filter in the flesh.

Cheerio,
Scott

Thanks Scott. That confirms what I have been thinking, that the craziness comes from the fact that the circuit is a resonator, not because it is HP or LP. This reminds me of the driven damped pendulum, which also is chaotic in certains regimes of operation. Quite possibly analagous. (Oh, and there has been a whole book written about this Wink )

The Fonik examples are wildly chaotic -- more that what I have been seeing with my version of Rene's circuit. I should probably try to figure out why. If we can find the right handles we may be able to make a triple-purpose module with filter, multiphase oscillator and VC chaos generator.

I'm still working on putting in the VC resonance. Yesterday turned out to be a bit frenetic for me, but I should have time to get it working today.

Thanks for your input. It has been very useful to me.

Very Happy

Ian
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crashlander42



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PostPosted: Wed Dec 12, 2007 9:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

I just breadboarded the MS20 filter from the schematic on Rene Schmitz's page and I found it very pleasing. Lots of nice squelchy resonance. Twisted Evil I think I'm going to assemble it in a permanent state tomorrow and toss it in my modular.

Earlier in this thread Ian mentions that the green LED's give it a "dirty" filtering action. I was curious (still learning this electronics magic) what propriety of the green diodes gives it that dirty filtering action, and why green specifically?

Many thanks to whoever feels like explaining that.

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frijitz



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PostPosted: Thu Dec 13, 2007 5:27 am    Post subject: Reply with quote  Mark this post and the followings unread

crashlander42 wrote:
Earlier in this thread Ian mentions that the green LED's give it a "dirty" filtering action. I was curious (still learning this electronics magic) what propriety of the green diodes gives it that dirty filtering action, and why green specifically?

As I mentioned in a later post, that observation was made with no level control on the input. So the rougher character was just because there was deeper clipping.

Very Happy

Ian
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frijitz



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PostPosted: Thu Dec 13, 2007 5:34 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm still confused a bit about levels. Can anybody tell me what the signal level is in the MS20 at the filter itself? And where on the knob does the oscillation start? In other words, how far beyond the onset of oscillation does the resonance control go.

Thanks!

Very Happy

Ian
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Luka



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PostPosted: Thu Dec 13, 2007 6:01 am    Post subject: Reply with quote  Mark this post and the followings unread

maybe download a "sneaky trial"* version of the korg legacy pack and have a go on the ms20 vst, it would be pretty close wouldnt it?







*not that i condone sneaky trial versions Wink
i own all my software btw
geez im getting all defensive about it hehe

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crashlander42



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PostPosted: Thu Dec 13, 2007 7:38 am    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:

As I mentioned in a later post, that observation was made with no level control on the input. So the rougher character was just because there was deeper clipping.


Ah. Sorry I missed that post. Rolling Eyes

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frijitz



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PostPosted: Thu Dec 13, 2007 10:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
Dang, how did I lose track of this thread? Yep, Ian - LP mode will do the same thing. You can have a fair amount of resonance going, remove the signal, and the filter will be self oscillating.

I worked out the differential equation for the nonlinear driven lopass. The nonlinearity is in the first derivative. I haven't seen this system discussed in the chaos literature, but it looks somewhat similar to one variant of the famous van der Pol equation. So the chaotic behavior seems plausible on theoretical grounds.

Very Happy

Ian
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PostPosted: Fri Dec 14, 2007 12:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

So...I'm just as curious - can anybody out there answer Ian's question about the MS-20 filters? Tom - you still have an MS-20, don't you?
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frijitz



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PostPosted: Sat Dec 15, 2007 6:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
So...I'm just as curious - can anybody out there answer Ian's question about the MS-20 filters? Tom - you still have an MS-20, don't you?

I'm surprised nobody seems to know this. Hellllllooooooooo.....

I asked Rene about it, and his guess is the levels are something around 2.5 to 3.5 V. He also believes that the Korg filter was set up so the max resonance was close to the oscillation point. This agrees with the schematic as posted by Tim, which shows a feedback gain of 2.2 vs the 2.0 needed for oscillation. So the original probably couldn't do the things you and Rene are doing with it (gain of 7). Was Rene the first to discover all this wild behavior?

In the mean time, I got through the theory for the HP and it comes out basically the same as for the LP (there's an extra but unimportant prefactor in the driving term). I think I have a simple way to make this closer to the van der Pol oscillator, so it may be interesting to see if this gives any new behavior.

Very Happy

Ian
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frijitz



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PostPosted: Sat Dec 15, 2007 7:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ah, found the spec sheet. Very Happy
http://www.korganalogue.net/korgms/images/service/ms20/specs.gif
Filter input is 3Vpp max. I.e., max amplitude of 1.5V. This is just as I expected. So the original Korg design looks very tame and ordinary. With Rene's version you not only have 3x the feedback gain, you have 4x the signal amplitude to feed back. Surprised

Very Happy

Ian
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PostPosted: Sat Dec 15, 2007 8:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Rene is always good for a surprise here and there. Very Happy

Weren't the green LEDs his way of saving on putting three diodes in series X 2, or does it go deeper than that? (IE, the specific voltage drop of green LEDs is what he was after).

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frijitz



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PostPosted: Sat Dec 15, 2007 9:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
Rene is always good for a surprise here and there. Very Happy

Weren't the green LEDs his way of saving on putting three diodes in series X 2, or does it go deeper than that? (IE, the specific voltage drop of green LEDs is what he was after).


Rene *is* good -- in general, not just here and there. Yes, the LEDs he used have almost exactly the same drop as 3 x diodes (~2V). The turn on is a bit sharper, but that isn't important.

So you two were the first to work with this ovedrdriven beastie? I've googled around a bit and don't see any other mention of all this weirdness.

Very Happy

Ian
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PostPosted: Sat Dec 15, 2007 10:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oh, yes, I was definitely using understatement there. Very Happy

He's an exceedingly intelligent guy, and a very kind person to boot. Heck, a good chunk of my built synthesizer is his stuff.

Really, this filter is all Rene, it came along long before I even knew him or even about him (I've just built and played with it). I think Allison Project has built one, and I'm sure there are others out there. Have you seen Dr. Joerg Schmitz's (no relation) page on it?

Back in January 2004 I visited Rene while on a business trip to Germany, he had so much cool stuff going on, it was almost mind-numbing. I remember he'd built a tube based phase shifter that was just the smoothest thing you'd ever hear. His synth was really interesting too - he'd built the panels out of some kind of blue plastic material - it looked really sharp.

Anyway, on that visit, I traded some parts for some of the MS-20 filter PCBs he'd had made up. I gave a couple of them to Jeff Pontius and kept one (it's in my module, the other one in the module is on protoboard).

Earlier, I was lucky enough to get three VCO3 boards he'd come up with, and those are the VCOs that are mainly heard in my samples. VCO3 has very wide range, which is an understatement. I coined the measurement term "coffebreak" (CB) in order to measure the thing at its lowest setting - I think it was somewhere around 1.5 CB/cycle Laughing . Earlier this fall, I was driving the Klee up to audio range with it, and when I looked at the frequency I was putting out, it was around 45 kHz, and it still had some room to go up. It's a real workhorse. The KTY part he used for temp compensation is a bit hard to come by on this side of the pond, though. Or, at least it was back then. It would be neat if some more of either of those PCBs could be made.

Cheerio,
Scott

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PostPosted: Sun Dec 16, 2007 8:19 am    Post subject: Reply with quote  Mark this post and the followings unread

I changed the name of the thread slightly to reflect that this version of the Late MS-20 Filter is René's design - it was around even when I began pestering René with a jillion of my newbie questions. I even got that little accent mark in there, which is something I can never quite remember how to do on the keyboard (I pasted it in there)! Notice it did take me a couple of weeks... Laughing

Cheerio,
Scott

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Phazinhead



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PostPosted: Tue Feb 12, 2008 10:21 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi everybody !


I'm new in this forum, I like it very much, there are a lot of intersting stuff to read and to discover Smile
I'm not very skilled in electronics, but I'm doing some DIY and I would like to build some filters now.. and as you can guess, I would like to try an MS20 filter clone. I've searched during long time on the internet for PCB and information, which led me to the MS20 filter version of Mr. Takeda and René Schmitz. René's version seems to be the easiest to build and to use standard european components, but I've still some stupid questions in mind .. so if you can help me Smile :

-What kind of potentiometer should I use for Frequency and Resonnance control ( value and type) ? (.. I found something that said 47K lin for frequency and 100K lin for reso but I'm not shure it is the right value for René's version)

-What are the references of the transistors to use ? Are there many reference of transistor available ?


Thank a lot for your help and your advice !
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crashlander42



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PostPosted: Sat Mar 08, 2008 12:04 am    Post subject: Reply with quote  Mark this post and the followings unread

I just put the Rene Schmitz version of this filter together on the bread board again because I'm planning on throwing one of these together on veroboard and tossing it in my modular.

For some reason the second time around the max resonance seems lower. It's considerably lower than it was the first time I built it. I remember the first time I built it being pleased with it's squelchyness.

Any idea where I would start poking at it to raise the max q?

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sneakthief



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PostPosted: Sun Mar 09, 2008 3:01 am    Post subject: Reply with quote  Mark this post and the followings unread

i built takeda's ms-20 clone and couldn't get it to work Sad it still sits there on my workbench, taunting me.
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bubblechamber



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PostPosted: Sun Mar 09, 2008 11:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Crash--- it's a 47k or 50k res pot. the resistor you'll want to play with for higher Q is the 10k between the out and neg of A3. also refer to the schematic here

http://www.electro-music.com/forum/viewtopic.php?highlight=ms20&t=20668

R16 in this schematic
it's a little different and it might work better for you. I think I got the best results with a 2.7K instead of the 10K. mine starts to self osc at around 90%
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crashlander42



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PostPosted: Mon Mar 10, 2008 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks! That helped. I also realized that without an attenuated input it's insanely overdriven and I lose a lot of the resonance. I think Ian mentioned something about that and it went in one ear and out the other.


Embarassed *edit* Embarassed
Ya. I completely missed the first half of the thread where they pretty much answered my question. The sad part is that I built this once before. Filter sounds great now.

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